`let` assigment breaks `typeof`

[mariorodriguespt]

• Version: v13.14.0
• Platform: Linux 5.3.0-51-generic README: fix prerequisite list formatting #44~18.04.2-Ubuntu SMP Thu Apr 23 14:27:18 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

What steps will reproduce the bug?

> let a = a || 2;
Uncaught ReferenceError: Cannot access 'a' before initialization
> typeof a !== 'undefined'
Uncaught ReferenceError: a is not defined
> typeof b !== 'undefined'
false

How often does it reproduce? Is there a required condition?

Every single time. If you use let to define a variable and in the assignment part you check if the same variable exists, you won't be able to use typeof after.

What is the expected behavior?

I would expect typeof should always work regardless of the variable a existing or not.

What do you see instead?

Uncaught ReferenceError: a is not defined

Additional information

This appear to be an issue in Chromium since it works as expected in Firefox.

[devsnek]

Duplicate of #8309, albeit a much more confusing one.

[devsnek]

Also, you've technically found a bug in firefox. If you feel like it you can report it here: https://bugzilla.mozilla.org/

[mariorodriguespt]

This is working well for me on Firefox 76.0.1 (64-bit)

Could you share your reproduction steps?

[devsnek]

@mariorodriguespt it should be throwing in firefox. This is the same case as { typeof x; let x; }

[mariorodriguespt]

It is not throwing on Firefox and it is correct.
It throws an error when you try let a = a || 2 but the typeof works as expected.

On your example, Firefox does throw an error: ReferenceError: "can't access lexical declaration x' before initialization"` which is correct due to the order.

As far as I can tell, V8 is not handling this correctly: The assignment failed but the typeof got broken, this issue is more relevant on a browser environment where you have a second script checking a var using typeof.

[devsnek]

When you do let x = /* something that throws an error */, the variable is created, but is never initialized. This state is called the "temporal dead zone" (https://snek.dev/blag/2019-07_js_hoisting might help). While a variable is in the temporal dead zone, typeof x will throw, because the variable exists and is resolvable, but getting its value to try to get the type of it throws a reference error. V8 is behaving correctly here, and firefox has a bug.

[mariorodriguespt]

Just opened a bug on Firefox, I'll leave the link for those interested in the follow up: https://bugzilla.mozilla.org/show_bug.cgi?id=1639078